1. Field of the Invention
The present invention relates to a method of recovering rare-earth elements involving using, as a raw material, a solid residue which is produced as a by-product in a Bayer process for separating and collecting an aluminum component in bauxite from the bauxite (The solid residue is hereinafter referred to as “bauxite residue.” A bauxite residue containing Fe2O3 as a main component has a red color and is generally called “red mud.”), and which contains Sc, Y, and lanthanoids, which belong to rare-earth elements, causing the rare-earth elements to leach from the bauxite residue, and separating and recovering the rare-earth elements.
2. Description of the Related Art
Rare-earth elements are widely used in applications such as a high strength Al alloy, a phosphor, a magnetic substance, optical glass, and a catalyst. Particularly in the magnetic substance, the use of the rare-earth elements as materials for producing a permanent magnet has been rapidly expanding because a magnet having a large maximum energy product and a large residual magnetic flux density can be obtained by adding the rare-earth elements to transition elements. For example, PTL (Patent Literature) No. 1 (JP 59-046,008 A) discloses materials for producing an Nd—Fe—B-based permanent magnet having an excellent maximum energy product and an excellent residual magnetic flux density. In addition, PTL No. 2 (JP 62-165,305 A) discloses a technology for improving the thermal stability of magnetic characteristics, which is a defect of the Nd—Fe—B-based permanent magnet, by substituting part of Nd with Dy in the permanent magnet.
For example, ores such as monazite, bastnaesite, xenotime, and ion adsorption clay mineral are used as raw materials for such rare-earth elements. The rare-earth elements are caused to leach from any of these ores by using an acidic aqueous solution, for example, an aqueous solution of a mineral acid such as sulfuric acid, and the rare-earth elements are separated and collected from the resultant leachate. However, these ore resources are unevenly distributed on the earth, and the abundance of each element in the rare-earth elements significantly varies for each ore. In particular, there are very few mines in which ores containing heavy rare-earth elements having atomic numbers of 64 to 71 and having high mine profitability can be mined, and it is concerned that the depletion of the resources of Dy, which is in especially high demand, may occur. Further, ores containing Sc alone are not mined as ores with good profitability, and tailings of, for example, U ores, which are raw materials for nuclear fuel, are used only as raw materials for Sc, and hence the production quantity of Sc is remarkably small.
On the other hand, the rare-earth elements are also contained in bauxite, which exists as a resource more abundantly than ores such as monazite, bastnaesite, xenotime, and ion adsorption clay mineral and which is an ore resource of aluminum. It is known that the rare-earth elements are caused to dissolve from bauxite and are then separated and recovered {see, for example, paragraph 0004 in PTL No. 3 (JP 09-176,757 A) and paragraph 0003 in PTL No. 4 (JP 09-184,028 A)}. Further, it is also known that, when aluminum is produced from bauxite through the steps of a Bayer process and Hall-Héroult process, rare-earth elements are caused to leach with sulfurous acid by using, as a raw material, a bauxite residue produced as a by-product in the Bayer process and are then separated and recovered {PTL No. 5 (U.S. Pat. No. 5,030,424)}. Further, there is known a technology involving causing Sc and lanthanoids to leach with nitric acid from such bauxite residue and separating and recovering them by an ion exchange method {NPTL (Non-Patent Literature) No. 1 (Ind. Eng. Chem. Res. 41(23), 5794-5801, “Pilot-Plant Investigation of the Leaching Process for the Recovery of Scandium from Red Mud”)}.
Bauxite contains aluminum oxide and ferric oxide as its main components. In the Bayer process for separating and collecting an aluminum component in bauxite, which serves as a raw material, from the raw material, aluminum oxide in the bauxite is dissolved as aluminum hydroxide in an alkaline aqueous solution of sodium hydroxide, and the aluminum hydroxide is caused to leach and is separated, thereby collecting the aluminum component in the raw material. Further, a bauxite residue produced as a by-product in this process contains, as a main component, ferric oxide, which does not react with an aqueous solution of sodium hydroxide. When bauxite contains rare-earth elements, the rare-earth elements exist as chemically stable compounds such as oxides or hydroxides in an aqueous solution of sodium hydroxide, and the compounds do not easily react with the aqueous solution of sodium hydroxide even when the aqueous solution is heated and pressurized. Thus, in the bauxite residue, the rare-earth elements are to be concentrated to the extent corresponding to the amount of the aluminum component caused to leach with the aqueous solution of sodium hydroxide in the Bayer process described above.
According to studies of the inventors of the present invention, the bauxite residue contains rare-earth elements about three times on the average in comparison to the content of rare-earth elements in bauxite. Further, the bauxite residue is an industrial waste which is produced as a by-product when aluminum is produced from bauxite, and hence can be easily obtained. Therefore, the bauxite residue is expected to be used as a raw material for rare-earth elements.
However, detailed examination of PTL No. 5 above has revealed that, as described in Examples 1 and 2 thereof, a bauxite residue containing, in dry basis, 52.0% of Fe2O3, 6.5% of TiO2, 18.0% of L.O.I, 12.9% of Al2O3, 2.4% of SiO2, 1.6% of Na2O, 5.0% of CaO, and 0.6% of P2O5 is used as a raw material, and a leaching (or digesting) operation is repeated two or three times between 10 and 70° C. performed for a sulfurous acid solution comprising the raw material and having a high pH value, by using a sulfurous acid solution having a low pH value to adjust the final pH value of the resultant solution to 1.35 to 2.4. Accordingly, rare-earth elements are caused to leach while keeping the dissolution of Fe and Ti contained in the bauxite residue at a low level, and the rare-earth elements are then separated and recovered by using a solvent extraction method. In this case, however, in a leaching time of 20 minutes, which is considered to be preferred to almost saturate the leaching amount of the rare-earth elements without continuously increasing the dissolution amount of Fe, about 65% of the content of Y in the bauxite leaches, while the leaching ratio of Nd is lower than that of Y and is only about 58% (see the descriptions on lines 32 to 36 in column 7, Tables 1 to 3, and FIG. 2 in PTL No. 5).
That is, the technology described in Examples 1 and 2 of the PTL No. 5 involves repeating the leaching operation two or three times, and hence, as the amount of a leachate increases, the cost of leaching treatment increases at the time of causing rare-earth elements to leach from a bauxite residue because, for example, it is required to repeat a solid-liquid separation operation two or three times. Moreover, when the liquid-solid ratios at the time of the leaching operations are compared between Example 1 (see Table 1) and Example 2 (see Table 3), the total leaching ratio of the first and second leaching operations in Example 1 is higher than that in Example 2. Digestion is carried out twice under the leaching conditions of 4:1 and 10:1, and the amount of a leachate becomes 14 times the amount of red mud serving as a raw material. Thus, it is required to use an extractant in a large amount corresponding to the amount of the leachate, the extractant being necessary in separation and recovery treatment for separating and recovering rare-earth elements from the leachate by the solvent extraction method. In addition, an expensive extractant such as EHEHPA is used. Accordingly, this technology has a problem in that the cost of the separation and recovery treatment becomes higher.
By the way, the inventors of the present invention used 0.102 kg of a bauxite residue having the same composition as that of the bauxite residue used in examples to be described below, and followed the experiment in Example 1 of PTL No. 5, which involves using an aqueous solution of sulfurous acid as an acidic aqueous solution and repeating the same extraction operation three times under the conditions of a liquid-solid ratio (L/S) of 5.0, a temperature of 30° C., a pressure of 0.1 MPa, and a time of 15 minutes. The results are as shown in Table 1. In the first leaching operation, the leaching ratio of Y merely reached 5 mass % or less, and the total leaching ratio of Y additionally including the leaching ratios of the second and third leaching operations was 52 mass %. However, the leaching ratios of Nd and Dy merely reached 41 mass % and 43 mass %, respectively, which were merely even lower values in comparison to the leaching ratio of Y.
TABLE 1Usage of bauxite residuekg0.102FirstKind of acidH2SO3leachingLiquid-solid ratio5.0LeachingTemperature° C.30conditionspHAfter completion of3.27leachingTimeMinutes15SecondKind of acidH2SO3leachingLiquid-solid ratio5.0LeachingTemperature° C.30conditionspHInitial stage of leaching2.05After completion of3.20leachingTimeMinutes15ThirdKind of acidH2SO3leachingLiquid-solid ratio5.0LeachingTemperature° C.30conditionspHInitial stage of leaching1.21After completion of1.82leachingTimeMinutes15pH valueInitial stage of leaching3.3After leaching1.2LeachingY52ratioNd41(mass %)Dy43Ca88Al40Si99Ti0.3Fe0.2
Further, NPTL No. 1 shows the leaching ratios of Sc, Y, and Fe resulting from the operation performed under the conditions of using 0.6 N HNO3 and adjusting the pH at the time of completion of leaching to about 0.15 to 0.44. The leaching ratios of Sc and Y sharply lowers as the pH at the time of completion of the leaching becomes higher, and, when the pH at the time of completion of the leaching is adjusted to 0.44, the leaching ratio of Y is about 38% (see FIG. 4). As in the case of PTL No. 5 described above, in NPTL No. 1 as well, it is described that the leaching operation needs to be repeated two or three times in order to increase the leaching ratios and that a liquid-solid ratio of 50 to 100, which is even higher than that in the case of PTL No. 4, is necessary. In addition, it is also described that, because the pH of the leachate is low, the dissolution ratio of Fe is as high as 2 to 4%. When the leaching ratios of impurities such as Fe become higher as described above, some problems occur, for example, it is required to use an extractant in a large amount corresponding to the amount of the leachate, the extractant being necessary in the subsequent steps, as in the case of PTL No. 5 described above.
According to NPTL No. 1, when rare-earth elements including Sc are caused to leach from a bauxite residue serving as a raw material and are recovered, the leaching operation is performed by using 0.6 N nitric acid under the conditions of a solid-liquid ratio (S/L) of 0.1 to 0.01 and a leaching time of 0.5 to 3 hours (see Table 2) because Sc is more difficult to dissolve in acid than lanthanoids. As the solid-liquid ratio (S/L) is smaller and as the leaching time is longer, the leaching ratio of rare-earth elements becomes higher, but, even in the case of run 5, in which the leaching ratio of Nd is high, the leaching ratios of Sc and Nd are 68.0% and 53.8%, respectively (see Table 3). Thus, NPTL No. 1 involves disadvantages such as the fact that the leaching ratio of Nd is not sufficiently high, the fact that the amount of Fe in the leachate in that case is 146.0×103 mg, which is equivalent to 100 times or more the amount of rare-earth elements, and the fact that the solid-liquid ratio needs to be adjusted to 0.01.